1. Field of the Invention
The present invention relates to a resonator, a filter, and a communication apparatus for use in, for example, radio communication in a microwave band or millimeter-wave band or transmission/reception of electromagnetic waves.
2. Description of the Related Art
In resonators using slot lines, design approaches that employ a step-impedance structure for the slot lines have been known for miniaturization of the resonators. Examples are described in Bharathi Bhat and Shiban K. Koul, “Analysis, Design and Applications of Fin Lines”, pp. 316–317, Artech House, Inc., U.S.A. 1987 and Yoshihiro Konishi, “Basics and Applications of Microwave Circuit (Maikuroha no Kiso to Ouyou)”, Sougou Denshi Syuppansya, pp. 169, 1990 (first edition). In the examples, the width in the vicinities of the opposite ends of the slot line is increased and the width of the center portion of the slot line is reduced, so that the impedance of the vicinities of the opposite ends of the slot line becomes inductive and the impedance of the center portion of the slot line becomes capacitive. Thus, the impedance in a direction along the slot line varies in a stepped manner, so that the length of the slot line needed for providing the same resonant frequency can be reduced.
FIGS. 16A and 16B show a typical example of such a known slot resonator having stepped impedance. FIG. 16B is a top view of a substrate having a slot resonator. FIG. 16A is a sectional view of the section A—A shown in FIG. 16B. A conductive film 10, which has conductor opening portions APa, APb, and APc, is provided on a surface of a dielectric substrate 1. The conductor opening portions APa, APb, and APc together define one dumbbell-shaped conductor opening portion. The widths of the conductor opening portions APa and APb (the widths can be called diameters in this case, because of their circular shapes) located at the opposite ends are relatively large, whereas the width of the center conductor opening portion APc is relatively small. As a result, the opposite ends of the dumbbell-shaped conductor opening portion have inductive impedance and the center portion has capacitive impedance.
The dotted lines in FIG. 16A schematically indicate the magnetic force lines of the slot resonator. The magnetic force lines represent the magnetic field distribution of the slot resonator. Thus, in the slot resonator having a stepped impedance structure, when a magnetic field vector is directed upward in one of the inductive regions located at the opposite ends, a magnetic field vector in the other inductive region is directed downward. As a result, the entire conductor opening portion behaves like a magnetic dipole. Much of magnetic field energy generated by the resonance is concentrated in inductive regions defined by the conductor opening portions APa and APb, and much of electric field energy is distributed along a capacitive region defined by the conductor opening portion APc. In this manner, the storing region of the magnetic field energy and the storing region of the electric field energy are separated from each other. Consequently, the conductor opening portion functions as a lumped element circuit, thereby making it possible to reduce the size of the slot resonator.
The slot resonator described above can be miniaturized due to its stepped impedance when it is configured to have the same resonant frequency. However, as the size of the resonator is reduced, the density of current flowing through the conductive film increases and thus the conductor loss increases. This poses a problem in that a resonator having a high unloaded Q-factor (Qo) cannot be provided.